JSON file for my Mist account?

I'm trying to transfer (import) my Mist account (address) into MetaMask, but MetaMask asks for a JSON file in order to do this. How do I find the JSON file for my Mist account? When I set up my Mist account, and to this day, I have never been able to set up a wallet contract due to an apparent software glitch, so I only have an "account", not a "wallet". Thanks